PWM buffer circuit for adjusting a frequency and a duty cycle of a PWM signal

ABSTRACT

A PWM buffer circuit includes a duty cycle converting circuit and a frequency-fixed PWM signal generating circuit. The duty cycle converting circuit is used for receiving a first PWM signal and then generating a duty cycle reference voltage on the basis of the first PWM signal. The duty cycle reference voltage is a one-to-one mapping function of the first duty cycle. The frequency-fixed PWM signal generating circuit is used for receiving the duty cycle reference voltage and then outputting a second PWM signal with a fixed frequency. The second PWM signal has a second duty cycle, which is determined in accordance with the duty cycle reference voltage. In addition, the second duty cycle is a one-to-one mapping function of the duty cycle reference voltage.

This application is a Continuation of co-pending application Ser. No.10/657,181 filed on Sept. 9, 2003, now U.S. Pat. No. 7,279,947, theentire contents of which are hereby incorporated by reference and forwhich priority is claimed under 35 U.S.C. §120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a buffer circuit applied with a pulsewidth modulation (PWM) signal and, more particularly, to a PWM buffercircuit for adjusting a frequency and a duty cycle of a PWM signal.

2. Description of the Related Art

In recent years, a fan for dissipating heat generally operates with afan motor whose speed is dominantly controlled through utilizing a PWMsignal. FIG. 1 is a circuit block diagram showing a control circuit forspeed of a fan motor by using a conventional PWM control method.Referring to FIG. 1, a PWM signal generation unit 10 outputs a PWMsignal S1 to a driving circuit 11. Based on the PWM signal S1, thedriving circuit 11 outputs a driving signal A to a fan motor 12, therebycontrolling the speed of the fan motor 12. More specifically, one ofsignal characteristics of the PWM signal S1 is known as “duty cycle,”i.e., a ratio of a pulse width to a period of the PWM signal S1. Assumethat the duty cycle of the PWM signal S1 in FIG. 1 is denoted by areference numeral D1. In the above-mentioned conventional PWM controlmethod, when the duty cycle D1 of the PWM signal S1 is relatively large,the driving signal. A output from the driving circuit 11 causes the fanmotor 12 to operate at a relatively high speed. On the other hand, whenthe duty cycle D1 of the PWM signal S1 is relatively small, the drivingsignal A output from the driving circuit 11 causes the fan motor 12 tooperate at a relatively low speed

However, the conventional PWM control method has at least twodisadvantages. The first one of the disadvantages is that the PWM signalS1 to be utilized must have a relatively high frequency, such as 10 kHzor more When the PWM signal S1 has a frequency lower than 10 kHz, theoperation of the fan motor 12 is adversely influenced by switchingnoise. The second disadvantage is that the duty cycle D1 of the PWMsignal S1 to be utilized must be restricted within a range between 30%and 85%, thereby ensuring that the driving circuit 11 and the fan motor12 can be appropriately controlled by the PWM signal S1.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, an object of the presentinvention is to provide a PWM buffer circuit, arranged in a controlcircuit for speed of a fan motor, for expanding an applicable range offrequency of a PWM signal to be used as a control signal.

Another object of the present invention is to provide a PWM buffercircuit, arranged in a control circuit for speed of a fan motor, forexpanding an applicable range of duty cycle of a PWM signal to be usedas a control signal.

According to one aspect of the present invention, a PWM buffer circuitincludes: a duty cycle converting circuit and a frequency-fixed PWMsignal generating circuit. The duty cycle converting circuit receives afirst PWM signal and then generates a duty cycle reference voltage basedon a first duty cycle of the first PWM signal. The duty cycle referencevoltage is a one-to-one mapping function of the first duty cycle. Thefrequency-fixed PWM signal generating circuit receives the duty cyclereference voltage and then outputs a second PWM signal having a fixedfrequency. The second PWM signal has a second duty cycle determined onthe basis of the duty cycle reference voltage, and the second duty cycleis a one-to-one mapping function of the duty cycle reference voltage.

According to another aspect of the present invention, a control circuitfor speed of a fan motor includes: a PWM signal generation unit, a PWMbuffer circuit, and a driving circuit. The PWM signal generation unitgenerates a first PWM signal having a first duty cycle. The PWM buffercircuit is coupled to the PWM signal generation unit for converting thefirst PWM signal into a second PWM signal having a fixed frequency and asecond duty cycle. The driving circuit is coupled to the PWM buffercircuit for outputting a driving signal based on the second PWM signalto the fan motor, thereby controlling the speed of the fan motor.

In one preferred embodiment of the present invention, the first PWMsignal has a frequency higher than 30 Hz and the first duty cycle islocated between 5% and 95%. Therefore, the PWM buffer circuit accordingto the present invention may be arranged in a control circuit for speedof a fan motor so as to expand an applicable range of frequency of thePWM signal as well as an applicable range of duty cycle of the PWMsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other objects, features, and advantages of thepresent invention will become apparent with reference to the followingdescriptions and accompanying drawings, wherein:

FIG. 1 is a circuit block diagram showing a control circuit for speed ofa fan motor by using a conventional PWM control method;

FIG. 2 is a circuit block diagram showing a control circuit for speed ofa fan motor arranged with a PWM buffer circuit according to the presentinvention;

FIG. 3 is a circuit block diagram showing a detailed configuration of aPWM buffer circuit according to the present invention;

FIG. 4( a) is a graph showing that a duty cycle reference voltage V1 isa one-to-one mapping function of a duty cycle D1 of a PWM signal S1;

FIG. 4( b) is a graph showing that a duty cycle D2 of a PWM signal S2 isa one-to-one mapping function of a duty cycle reference voltage V1; and

FIG. 5 is a diagram showing one example of a PWM buffer circuitaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments according to the present invention will bedescribed in detail with reference to the drawings.

FIG. 2 is a circuit block diagram showing a control circuit for speed ofa fan motor arranged with a PWM buffer circuit 20 according to thepresent invention. Referring to FIG. 2, the present invention isdifferent from the prior art shown in FIG. 1 in that a PWM buffercircuit 20 is arranged between the PWM signal generation unit 10 and thedriving circuit 11 such that a PWM signal S1 output from the PWM signalgeneration unit 10 is firstly converted into a PWM signal S2, which issubsequently input into the driving circuit 11. Based on the PWM signalS2, the driving circuit 11 outputs a driving signal B to the fan motor12.

More specifically, the PWM buffer circuit 20 converts the PWM signal S1having the duty cycle D1 and the frequency F1 into the PWM signal S2having a duty cycle D2 and a frequency F2. In the present invention, theduty cycle D2 and the frequency F2 of the PWM signal S2 are designed tohave values that ensure an appropriate control for speed of the fanmotor without causing any switching noise. Therefore, with such aconfiguration, even when the duty cycle D1 and the frequency F1 of thePWM signal S1 do not fall in a range which ensures an appropriatecontrol for speed of the fan motor, an appropriate control for speed ofthe fan motor is still achieved without causing any switching noisebecause the driving circuit 11 receives the PWM signal S2, which is theconverted signal from the PWM signal S1 by the PWM buffer circuit 20. Inother words, the PWM buffer circuit 20 according to the presentinvention is arranged in the control circuit for speed of the fan motorso as to expand the applicable range of frequency of the PWM signal aswell as the applicable range of duty cycle of the PWM signal.

As described in the conventional PWM control method shown in FIG. 1, thefrequency of the PWM signal S1 must be higher than 10 kHz and the dutycycle D1 thereof must be restrained between 30% and 85%. However, in oneembodiment of the present invention, the PWM buffer circuit 20 isdesigned to convert a PWM signal S1, which has a frequency higher than30 Hz and a duty cycle located between 5% and 95%, into a PWM signal S2,which has a frequency F2 higher than 10 kHz and a duty cycle D2 locatedbetween 30% and 85%. Consequently, the applicable frequency of the PWMsignal S1 is expanded to any of values higher than 30 Hz and theapplicable duty cycle thereof is expanded to any of values between 5%and 95% through utilizing the PWM buffer circuit 20 according to thepresent invention.

FIG. 3 is a circuit block diagram showing a detailed configuration of aPWM buffer circuit 20 according to the present invention. Referring toFIG. 3, the PWM buffer circuit 20 includes a duty cycle convertingcircuit 21 and a frequency-fixed PWM signal generating circuit 22. Morespecifically, the duty cycle converting circuit 21 receives the PWMsignal S1 and then generates a duty cycle reference voltage V1 based onthe duty cycle D1 of the PWM signal S1. In other words, the duty cyclereference voltage V1 is a one-to-one mapping function of the duty cycleD1 of the PWM signal S1, as shown in FIG. 4( a). The frequency-fixed PWMsignal generating circuit 22 receives the duty cycle reference voltageV1 and then determines the duty cycle D2 of the PWM signal S2 based onthe duty cycle reference voltage V1. In other words, the duty cycle D2of the PWM signal S2 is a one-to-one mapping function of the duty cyclereference voltage V1, as shown in FIG. 4( b). To sum up, in order toconvert the duty cycle D1 into the duty cycle D2, the PWM buffer circuit20 converts, at a first stage, the duty cycle D1 into the duty cyclereference voltage V1 by utilizing the duty cycle converting circuit 21,followed by converting the duty cycle reference voltage V1 into the dutycycle D2 by utilizing the frequency-fixed PWM signal generating circuit22 at a second stage.

In addition, the frequency-fixed PWM signal generating circuit 22generates a PWM signal S2 with a fixed frequency regardless of magnitudeof the duty cycle reference voltage V1. Accordingly, the frequency-fixedPWM signal generating circuit 22 is designed to output a PWVM signal S2with a frequency F2 that is high enough for avoiding switching noise.

In one embodiment of the present invention, the frequency-fixed PWMsignal generating circuit 22 may be implemented by a microchip controlunit, which is set through software programs to perform the desiredfunctions according to the present invention. In another embodiment ofthe present invention, the frequency-fixed PWM signal generating circuit22 includes a frequency controller 23 and a PWM signal generator 24, asshown in FIG. 3. More specifically, the frequency controller 23 providesa frequency control signal FC for determining the frequency of the PWMsignal S2 generated by the PWM signal generator 24. Based on the dutycycle reference voltage V1 from the duty cycle converting circuit 21 andthe frequency control signal FC from the frequency controller 23, thePWM signal generator 24 generates the PWM signal F2 having the dutycycle D2 and the frequency F2.

FIG. 5 is a diagram showing one example of a PWM buffer circuit 20according to the present invention. Referring to FIG. 5, the duty cycleconverting circuit 21 includes a transistor Q1, a plurality of resistorsR1 to R5, a diode Dd1, a capacitor C1, and an operational amplifier OA1.The frequency controller 23 includes a plurality of resistors R6 to R8,a capacitor C2, and an operational OA2. The PWM signal generator 24includes an operational amplifier OA3 and a resistor R9.

More specifically the transistor Q1 has a gate for receiving the PWMsignal S1, a drain coupled to a voltage source V_(DD) through theresistor R1, and a source coupled to ground. The diode Dd1 has a Pelectrode electrically connected to the drain of the transistor Q1 andan N electrode electrically connected to a non-inverting input terminalof the operational amplifier OA1. The resistor R2 and capacitor C1 areboth electrically connected between the N electrode of the diode Dd1 andthe ground. The resistor R3 is electrically connected between aninverting input terminal of the operational amplifier OA1 and theground. The resistor R4 is electrically connected between an outputterminal of the operational amplifier OA1 and the ground. The outputterminal of the operational amplifier OA1 outputs the duty cyclereference voltage V1 through the resistor R5 to a non-inverting inputterminal of the operational amplifier OA3.

The resistor R6 is electrically connected between an inverting inputterminal of the operational amplifier OA2 and the ground. The resistorR7 is electrically connected between the inverting input terminal of theoperational amplifier OA2 and an output terminal of the operationalamplifier OA2. The capacitor C2 is electrically connected between anon-inverting input terminal of the operational amplifier OA2 and theground. The resistor R8 is electrically connected between thenon-inverting input terminal of the operational amplifier OA2 and theoutput terminal of the operational amplifier OA2. With such aconfiguration, the output terminal of the operational amplifier OA2outputs the frequency control signal FC through the resistor R8 to theinverting input terminal of the operational amplifier OA3. In theexample shown in FIG. 5, the frequency control signal FC is a continuoustriangular wave signal having a frequency f as expressed in Equation(1):

$\begin{matrix}{f = \frac{1}{2R_{8}C_{2}{\ln\left( {1 + {2\;\frac{R_{6}}{R_{7}}}} \right)}}} & (1)\end{matrix}$

In response to the duty cycle reference voltage V1 received at thenon-inverting input terminal of the operational amplifier OA3 and thefrequency control signal FC received at the inverting input terminal ofthe operational amplifier OA3, the operational amplifier OA3 outputs thePWM signal S2 from an output terminal through the resistor R9. Morespecifically, the operational amplifier OA3 works as a voltagecomparator such that the operational amplifier OA3 outputs a higherlevel state of the PWM signal S2 when the duty cycle reference voltageV1 is larger than the voltage level of the frequency control signal FCwhereas the operational amplifier OA3 outputs a lower level state of thePWM signal S2 when the duty cycle reference voltage V1 is smaller thanthe voltage level of the frequency control signal FC. In such a manner,the PWM signal generator 24 converts the duty cycle reference voltage V1into the duty cycle D2. In addition, the PWM signal S2 generated by thePWM signal generator 24 has a frequency F2, which is equal to thefrequency f of the frequency control signal FC.

While the invention has been described by way of examples and in termsof preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications. Therefore, the scope of theappended claims should be accorded the broadest interpretation so as toencompass all such modifications.

1. A pulse width modulation (PWM) buffer circuit for expanding anapplicable range of frequency or duty cycle of a first PWM signal,comprising: a duty cycle converting circuit for receiving the first PWMsignal and then generating a duty cycle reference voltage based on afirst duty cycle of the first PWM signal, wherein the duty cyclereference voltage is a one-to-one mapping function of the first dutycycle, wherein the duty cycle converting circuit comprises: a transistorfor receiving the first PWM signal; a first resistor connected betweenof the transistor and a voltage source; a diode connected to thetransistor; a second resistor connected between the diode and theground; a first capacitor connected between the diode and the ground; afirst operational amplifier connected to the diode; a third resistorconnected between the first operational amplifier and the ground; afourth resistor connected between an inverting input terminal of thefirst operational amplifier and an output terminal of the firstoperational amplifier; and a fifth resistor connected between the firstoperational amplifier and the frequency-fixed PWM signal generatingcircuit; and a frequency-fixed PWM signal generating circuit coupled tothe duty cycle converting circuit for receiving the duty cycle referencevoltage and then outputting a second PWM signal having a fixedfrequency, wherein the second PWM signal has a second duty cycledetermined on the basis of the duty cycle reference voltage, and thesecond duty cycle is a one-to-one mapping function of the duty cyclereference voltage.
 2. The PWM buffer circuit according to claim 1,wherein the frequency-fixed PWM signal generating circuit is implementedby a microchip control unit set through software programs.
 3. The PWMbuffer circuit according to. claim 1, wherein the frequency-fixed PWMsignal generating circuit comprises: a frequency controller forproviding a frequency control signal to determine the fixed frequency ofthe second PWM signal; and a PWM signal generator coupled to the dutycycle converting circuit and the frequency controller for generating thesecond PWM signal in response to the duty cycle reference voltage andthe frequency control signal.
 4. The PWM buffer circuit according toclaim 3, wherein the frequency controller comprises: a first operationalamplifier; a first resistor connected between the first operationalamplifier and the ground; a second resistor connected between anon-inverting input terminal of the first operational amplifier and anoutput terminal of the first operational amplifier; a first capacitorconnected between the first operational amplifier and the ground; and anthird resistor connected between the non-inverting input terminal of thefirst operational amplifier and the output terminal of the firstoperational amplifier.
 5. The PWM buffer circuit according to claim 3,wherein the PWM signal generator comprises: a first operationalamplifier connected to the duty cycle converting circuit for receivingthe duty cycle reference voltage and connected to the frequencycontroller for receiving the frequency control signal, and a firstresistor connected to the first operational amplifier and outputting thesecond PWM signal.
 6. The PWM buffer circuit according to claim 3,wherein the frequency control signal is a continuous triangular wavesignal.
 7. The PWM buffer circuit according to claim 1, wherein afrequency of the first PWM signal is higher than 30 Hz and the firstduty cycle is located between 5% and 95%, and the fixed frequency of thesecond PWM signal is higher than 10 kHz.
 8. A control circuit for speedof a fan motor, comprising: a PWM signal generation unit for generatinga first PWM signal having a first duty cycle; a PWM buffer circuit forexpanding an applicable range of frequency or duty cycle of the firstPWM signal, coupled to the PWM signal generation unit for converting thefirst PWM signal into a second PWM signal having a fixed frequency and asecond duty cycle, wherein the PWM buffer circuit comprises: a dutycycle converting circuit for receiving the first PWM signal and thengenerating a duty cycle reference voltage based on the first duty cycleof the first PWM signal, wherein the duty cycle reference voltage is aone-to-one mapping function of the first duty cycle, wherein the dutycycle converting circuit comprises: a transistor for receiving the firstPWM signal; a first resistor connected between the transistor and avoltage source; a diode connected to the transistor; a second resistorconnected between the diode and the ground; a first capacitor connectedbetween the diode and the ground; a first operational amplifierconnected to the diode; a third resistor connected between the firstoperational amplifier and the ground; a fourth resistor connectedbetween an inverting input terminal of the first operational amplifierand an output terminal of the first operational amplifier; and a fifthresistor connected between the first operational amplifier and thefrequency-fixed PWM signal generating circuit and a frequency-fixed PWMsignal generating circuit coupled to the duty cycle converting circuitfor receiving the duty cycle reference voltage and then outputting, thesecond PWM signal, wherein the second duty cycle of the second PWMsignal is determined on the basis of the duty cycle reference voltage,and the second duty cycle is a one-to-one mapping function of the dutycycle reference voltage; and a driving circuit coupled to the PWM buffercircuit for outputting a driving signal based on the second PWM signalto the fan motor, thereby controlling the speed of the fan motor.
 9. Thecontrol circuit according to claim 8, wherein the frequency-fixed PWMsignal generating circuit is implemented by a microchip control unit setthrough software programs.
 10. The control circuit according to claim 8,wherein the frequency-fixed PWM signal generating circuit comprises: afrequency controller for providing a frequency control signal todetermine the fixed frequency of the second PWM signal; and a PWM signalgenerator coupled to the duty cycle converting circuit and the frequencycontroller for generating the second PWM signal in response to the dutycycle reference voltage and the frequency control signal.
 11. Thecontrol circuit according to claim 10, wherein the frequency controllercomprises: a first operational amplifier; a first resistor connectedbetween the first operational amplifier and ground; a second resistorconnected between a non-inverting input terminal of the firstoperational amplifier and an output terminal of the first operationalamplifier; a first capacitor connected between the first operationalamplifier and the ground; and an third resistor connected between thenon-inverting input terminal of the first operational amplifier and theoutput terminal of the first operational amplifier.
 12. The controlcircuit according to claim 10, wherein the PWM signal generatorcomprises: a first operational amplifier connected to the duty cycleconverting circuit for receiving the duty cycle reference voltage, andconnected to the frequency controller for receiving the frequencycontrol signal; and a first resistor connected to the first operationalamplifier and outputting the second PWM signal.
 13. The control circuitaccording to claim 10, wherein the frequency control signal is acontinuous triangular wave signal.
 14. The control circuit according toclaim 8, wherein a frequency of the first PWM signal is higher than 30Hz and the first duty cycle is located between 5% and 95%.
 15. Thecontrol circuit according to claim 8, wherein the fixed frequency of thesecond PWM signal is higher than 10 kHz.